Contribution of different grass species to plant-atmosphere ammonia exchange in intensively managed grassland

Abstract

Species diversity in grasslands usually declines with increasing input of nitrogen from fertilizers or atmospheric deposition. Conversely, species diversity may also impact the build-up of soil and plant nitrogen pools. One important pool is NH₃/NH₄⁺ which also can be exchanged between plant leaves and the atmosphere. Limited information is available on how plant-atmosphere ammonia exchange is related to species diversity in grasslands. We have here investigated grass species abundance and different foliar nitrogen pools in 4-year-old intensively managed grassland. Apoplastic pH and NH₄⁺ concentrations of the 8 most abundant species (<i>Lolium perenne, Phleum pratense, Festuca pratensis, Lolium multiflorum, Poa pratensis, Dactylis glomerata, Holcus lanatus, Bromus mollis</i>) were used to calculate stomatal NH₃ compensation points. Apoplastic NH₄⁺ concentrations differed considerably among the species, ranging from 13 to 117 μM, with highest values in <i>Festuca pratensis</i>. Also apoplastic pH values varied, from pH 6.0 in <i>Phleum pratense</i> to 6.9 in <i>Dactylis glomerata</i>. The observed differences in apoplastic NH₄⁺ and pH resulted in a large span of predicted values for the stomatal NH₃ compensation point which ranged between 0.20 and 6.57 nmol mol^&minus;1. Three species (<i>Lolium perenne, Festuca pratensis</i> and <i>Dactylis glomerata</i>) had sufficiently high NH₃ compensation point and abundance to contribute to the bi-directional NH₃ fluxes recorded over the whole field. The other 5 grass species had NH₃ compensation points considerably below the atmospheric NH₃ concentration and were thus not likely to contribute to NH₃ emission but only to NH₃ uptake from the atmosphere. Evaluated across species, leaf bulk-tissue NH₄⁺ concentrations correlated well (<i>r</i>²=0.902) with stomatal NH₃ compensation points calculated on the basis of the apoplastic bioassay. This suggests that leaf tissue NH₄⁺ concentrations combined with data for the frequency distribution of the corresponding species can be used for predicting the NH₃ exchange potential of a mixed grass sward.